1. Technical Field
This invention relates generally to biasing circuits and, in particular, to constant current biasing circuits.
2. Related Art
A resistor and a diode are commonly used with a bias voltage to create a biasing current for use in numerous types of circuits including amplifiers. As the bias voltage is applied to the resistor diode biasing circuit, a biasing current results. A disadvantage of using a diode in the biasing circuit is that the diode does not create a linear relationship between the bias voltage and the biasing current. Additionally, the dynamic range of the biasing current is limited by a resistor diode biasing circuit.
The diode in the resistor diode biasing circuit also results in biasing current fluctuations when the temperature changes. The biasing current fluctuations have an adverse effect on the circuit being biased and must be tolerated or have additional temperature compensation circuitry added. Thus, there is a need in the art for a biasing circuit that has a linear relationship between the biasing voltage and biasing current and that functions over a broad dynamic range and is unaffected by temperature changes.
Broadly conceptualized, the invention is a constant current biasing circuit that has a proportional relationship between the biasing voltage and biasing current and that is unaffected by temperature changes. The constant current biasing circuit may be implemented in a complementary metal oxide semiconductor (CMOS) in order to take advantage of the electronic characteristics of CMOS-fabricated circuits.
An example implementation of a constant current biasing circuit is in a linear amplifier. A biasing voltage results in a biasing current that may then be mirrored as a reference current to the power amplifier. The bias circuit also provides feedback around a reference transistor residing on the power amplifier. This feedback loop maintains a quiescent bias for the reference transistor equal to the reference current. The resulting bias voltage from the feedback loop is then used to bias the RF power device. Since the bias point of the amplifier is controlled and does not vary with temperature, the amplifier exhibits reduced variation in linearity over temperature. As a result, the amplifier benefits from increased design margin.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following FIGS. and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.